1. Field of the Invention
The present invention relates to a lifetime evaluating system of material of a mechanical element and a method for the same.
2. Description of the Related Art
A machine such as a turbine of a thermal power generation plant has a machine element which operates at high temperature and in which the start and stop of the operation are frequently repeated. Such a mechanical element has a finite lifetime due to thermal fatigue and creep. Therefore, it is important to maintain the plant to allow the operation for a long term. Especially, the extension of durable years is important economically. For this purpose, a lifetime evaluation technique has been developed and requires high reliability.
A conventional lifetime evaluation is carried out through a metallographic check by an expert. As a non-destruction lifetime diagnosing method in which the metallographic change of metal material is evaluated, a metallographic comparison method is known. The metallographic comparison method has been developed as the lifetime evaluating method of brittle creep degradation in a welding portion of low alloy steel having a martensite structure. The metallographic comparison method is evaluated in high precision and is widely used as a periodical test method.
In the metallographic comparison method, as shown in FIG. 1, the metallographic structure of a test object portion of a mechanical element is copied as a replica 101. Also, a comparison table 109 is produced through the observation of creep cavities 103 by a scanning type electron microscope 102, the observation of a micro crack 105 and metallographic structure 106 by an optical microscope 104, and the observation of precipitation product 108 by an analysis electron microscope 107. In the comparison table 109, the damage state of the test object portion is classified into mechanical damage, optical microscopic metallographic damage, and precipitation product distribution damage based on damage factors. The optical microscopic metallographic damage and the precipitation product distribution damage contain metallographic changes such as the recovery of the martensite structure in the first half of the lifetime of the mechanical element and changes of a precipitation product distribution. The mechanical damage contains a mechanical change such as generation, combination and growth of creep voids and micro cracks in the second half of the lifetime. The degradation of the test object portion is determined based on a combination of three damage states. Each damage state is classified into classes. The classification is carried out based on the comparison of a sample with a standard specimen. The classes of the degradation correspond to the lifetime consumed percentages shown in the comparison table 109. The classification and comparison are carried out by the expert with material knowledge.
The classification with respect to the microscopic structure is exemplified as follows. A welding portion of CrMo steel is formed to have the martensite structure. When stress is applied to the martensite structure at high temperature, the structure is changed through the creep transformation. Such a change process is classified into degradation classes IM, IIM, and IIIM by the expert, as shown in the column of the microscopic structure of the comparison table 109. The degradation class IM is equivalent to an unused state, in which a line structure called martensite lath can be clearly observed in a grain. The lifetime consumed percentage of the welding portion is 0–20%. The degradation class IIM is a state, in which carbide is precipitated in the neighborhood of the boundary of martensite lath. The lifetime consumed percentage of the welding portion is 20–40%. The degradation class IIIM is a state, in which the martensite lath disappears, and carbide is precipitated in old austinite grain boundary. The lifetime consumed percentage of the welding portion is 40–100%. As mentioned above, the point to which the expert pays the most of attention in the classification of the degradation is the disappearance of the martensite laths. The line structure can be clearly confirmed in a non-damaged state, but becomes unclear gradually with the use time of the mechanical element. The expert determines the degradation based on a rate of martensite laths.
The classification with respect to the mechanical damage is exemplified as follows. The mechanical damage of the welding portion of CrMo steel is classified by the expert through quantification of creep voids generated as the creep damage progresses and micro cracks formed by combination of the creep voids. The mechanical damage is classified into four degradation classes ID, IID, IIID, and IVD. The degradation class ID is a state, in which no creep void is generated or independent creep voids are generated. The lifetime consumed percentage is 0–50%. The degradation class IID is a state, in which the creep voids concatenate to each other, and the lifetime consumed percentage is 50–75%. The degradation class IIID is a state, in which the creep voids concatenate along the whole boundary of the one crystal grain to generate a micro crack. The lifetime consumed percentage is 75–80%. The degradation class IVD is s state, in which the creep voids concatenate over the boundaries of the two or more crystal grains to generate the micro cracks. The lifetime consumed percentage is 80–100%.
This evaluating method strongly depends on the human ability of the expert and it is difficult to cope with the rapid increase of the lifetime evaluation object plants. Moreover, it is necessary to bring the sample or the data from the plant installation sites back to a laboratory, resulting in requirement of a lot of time.